Mineral control of carbon pools in a volcanic soil horizon

The aim of this study was to characterize carbon sequestration by mineralogical control at the scale of a volcanic soil horizon. We adapted the classical density fractionation procedure and focused on the heavy fractions (> 1.9), which we divided into eight organomineral fractions. We characterized them simultaneously through non-destructive mineralogical analyses (XRD and NMR of Al and Si) and organic carbon analyses. The results showed that the largest proportion (82.6%) of organic matter in the horizon was associated with minerals in organomineral complexes. Imogolite type materials bound 6-fold more OM than anorthoclase, and 3.5-fold more OM than iron oxides. In addition, we observed a degree of polymerization of imogolite type materials that was midway between that of allophane and Al in Al-humus complexes. In conclusion, the results of this density fractionation combined with a mineralogical approach suggested that OM in the heavy fractions could be divided into several pools depending on the nature of the minerals.